In electrochemical sensors having co-planar electrode configurations, unintentional electrical shorts between electrodes can lead to inaccurate estimation or calculation of an amount of an analyte in a sample fluid. Typically this is avoided by conducting certain failsafe system checks on the sensor, such as by the analytical device (meter) to which the sensor corresponds. Common checks include measuring continuity between pairs of electrodes where unintentional shorts can cause inaccurate measurement results. If continuity between electrodes is detected or measured by the circuitry of the meter when such continuity should not exist, the meter displays an error signal and the strip is not used.
Advances in electrochemical sensors, however, have resulted in more complex electrode systems, often comprising three, four, five, even up to between ten and fifteen different electrodes. While measuring continuity between various pairs of electrodes in a system of two, three or even four electrodes can be simple and easy to implement, more complex multiple electrode systems would require much more complex meter programming, including algorithms and/or logic statements and rules. As a result, this simple failsafe may be less practicable to implement.
One particularly undesirable short circuit in a co-planar electrochemical sensor is between a working electrode and any other electrode that may come in contact with a fluid sample applied to the sensor. Typically, the electrochemical response of the analyte in the fluid sample is proportional to the surface area of the working electrode in contact with the sample. In certain sensors, such as capillary channel fill sensors, one or more sample sufficiency electrodes may be provided in a downstream location in order to detect a sufficient fill level of the sample in the sample channel. If one or more such sufficiency electrodes is shorted to the working electrode, then the working electrode's surface area is effectively increased by the amount of the sufficiency electrode in contact with the fluid sample. Relatively accurate estimation or calculation of the concentration of the targeted analyte depends in part on a generally constant value of working electrode surface area through which the current generated from the predetermined reaction flows. Thus, the increased working electrode surface area that is caused by the undesirable short circuit produces a higher concentration measurement result.
It is generally known, for example in a Therasense Freestyle electrochemical sensor, to provide any other electrodes that come in contact with the fluid sample with generally smaller surface areas than the working electrode. In the past, however, this is done only in sensors comprising a facing (or opposing) electrode configuration. Generally, the intent of such a design is to provide a large counter electrode that does not limit the current induced by the electrochemical reaction at the working electrode, and to assure that the sample chamber of the electrochemical sensor is completely filled before a measurement sequence is initiated. Incidentally, it is as a result of the facing configuration that there is a reduced likelihood of undesirable shorts, and the probability of a harmful short circuit is much less in the facing configuration.